The present invention relates to a bending method of a pipe material.
As a technique of bending a pipe material (a work) at a significantly small bending radius, xe2x80x9cWorking Technique of Bending Pipe at Radius 0xe2x80x9d is disclosed in JOURNAL OF THE JAPAN SOCIETY FOR TECHNOLOGY OF PLASTICITY Vol. 35 No. 398 pages 341 to 346.
This prior art relates to a method of continuously bending a pipe in a shearing direction by applying an internal pressure p to the pipe (a pipe material) 1A held within two sets of clamps 2A and 3A by an oil 4A and by sliding both of clamps with each other on a boundary surface, as shown in FIGS. 10A to 10C, whereby a bent portion at a minimum radius can be obtained. A crank-like bent product is used as it is or is separated into two parts having the same shape for obtaining two pieces at the same time so as to be used.
In another prior art, Japanese Patent Unexamined Publication No. 6-238352, as shown in FIGS. 11A and 11B, there is disclosed a method of bending a metal pipe at a small radius of curvature in accordance with a mechanical structure by inserting a mandrel 4B having a front end portion formed in a curved surface (a guide surface) shape into a pipe 1B and relatively rotating both of the clamps 2B and 3B around a boundary line.
In the former of the prior arts mentioned above, since the bent portion is the minimum radius obtained by substantially perpendicularly bending, there is a problem that the bent portion increases a fluid resistance in the case of using the bent product as a fluid pipe. Further, when using it as a structural member, there is a problem that a stress concentration is generated in the bent portion so as to form a breakage start point. Further, since the internal pressure is applied by the oil, there is a problem that a pressure application apparatus is required. Further, since steps of charging and discharging the oil are required, there is a problem that the apparatus is not suitable for a mass production.
Further, in the latter of the prior arts mentioned above, since it is necessary to make a rotatable exclusive clamp, there are problems that it is disadvantageous in a manufacturing cost and a set-up time, or it is impossible to obtain two pieces at the same time.
An object of the present invention is to provide a bending method of a pipe material which can solve the problems mentioned above.
In order to achieve the objects mentioned above, in accordance with the present invention, there is provided a bending method of a pipe material comprising the steps of:
firmly holding a pipe material by a first clamp as well as loosely holding by a second clamp;
arranging mandrels inserted from both ends of the pipe material within the pipe material in such a manner as to be brought into contact with a portion near a boundary surface of both of the clamps or closely oppose to the portion, thereby fixing it so as not to relatively move with respect to both of the clamps; and
relatively moving both of the clamps in parallel to the boundary surface in an opposite direction to each other, while maintaining the fixing state, whereby two bent portions and an intermediate straight portion are formed in the pipe material.
Both of the mandrels are firmly fixed so that the position with respect to the clamps does not move until the bending process is finished, after being arranged and fixed. This is the reason for maintaining a clearance (an interval at which the pipe material is plastically flowed) between the mandrel and the clamp constant.
Further, in accordance with the present invention, with respect to the mandrel, the structure may be made such as to insert a front end of a first mandrel inserted to the first clamp via the pipe material into the second clamp, to insert a front end of a second mandrel inserted to the second clamp via the pipe material into the first clamp, and to constitute the front end portions of the respective mandrels by flat surfaces including a pipe axis and a curved surface being convex to a side opposite to the relative moving direction at a time of processing of the clamp to which the front end is inserted, both flat surfaces being brought into contact with or closely opposed to each other at a time of process starting, and to form a space in which the front ends of both of the mandrels can relatively move due to a relative movement of both of the clamps, in both of the clamps.
Further, in accordance with the present invention, it is possible to bend the pipe material while applying a pressing force in an axial direction to the pipe material.
The pressing force applied to the pipe material serves to assist a plastic fluidization of the pipe material at a time of bending.
Further, in the structure mentioned above, the relative movement of both of the clamps and the pressing force of the pipe material close to the second clamp may be obtained by a common power.
Further, in accordance with the present invention, the structure may be made such that both of the clamps and a slider are arranged between an upper table and a lower table which can relatively move close to each other and apart from each other, a relative movement of both of the clamps and a relative movement of the slider are achieved by an approaching force between the upper table and the lower table, and the pipe material close to the second clamp is pressed in accordance with the relative movement of the slider.